1. Field of the Invention
The present invention is directed to a phosphor having reduced afterglow, suitable for use in detecting high energy radiation, such as x-rays, and to a method for manufacturing such a phosphor.
2. Description of the Prior Art
Detectors for high energy radiation can be constructed of a phosphor and a photodiode or a photomultiplier. Such detectors have widespread employment in nuclear medicine and in x-ray diagnostics. The phosphor has the function of absorbing the high-energy radiation and emitting visible light as a consequence of this absorption. This visible light can be detected by a photosensitive element, for example a photodiode, a photomultiplier or a light-sensitive film.
Phosphors having only an extremely slight afterglow are required in modern radiation detectors as employed, for example, in x-ray computed tomography in order to achieve an adequately high read-out frequency. Thallium-doped cesium iodide CsI:TI is a phosphor in widespread use that, for example, still exhibits an afterglow intensity of approximately 10.sup.-2 through 10.sup.-3 of the initial light intensity, for example 30 msec after the high-energy radiation has ceased to be incident on the phosphor. For modern radiation detectors, however, phosphors are required whose afterglow has dropped to less than 10.sup.-4 of the initial intensity after approximately 5 through 10 msec.
Promising phosphors for employment in modern radiation detectors are considered to be the oxisulfides of the rare earths. German OS 36 29 180 discloses a method for manufacturing a phosphor ceramic having the general composition (Ln.sub.1-x-y M.sub.x Ce.sub.y).sub.2 O.sub.2 S,X, with Ln.dbd.Gd, La or Y; M.dbd.Eu,Pr or Tb and X.dbd.32 F or Cl with 0&lt;x, y&lt;1.
The pigment powder employed as initial material is thereby filled into a vacuum-tight metal container and is compressed to form a ceramic by isostatic hot-pressing.
It is proposed in an article in J. Electrochem. Soc., Vol. 136, No. 9, September 1989, pages 2713 ff to dope a phosphor ceramic of a rare earth oxisulfide with cerium in order to reduce the afterglow. The light yield of the phosphor is reduced, however, at the same time due to the cerium additive and the phosphor is thus degraded in terms of another important property.